UNDERTHRUSTING 347 



rotation on a horizontal axis deep in the earth's crust, as shown in the 

 diagram, figure 6, in which AB is the surface of the earth, CD the rup- 

 ture, and the axis of rotation. The depth of is determinable if the 

 curvature of CD is known. As displacement proceeds, the vertical com- 

 ponent of movement at D is small and the horizontal component large, 

 while the reverse is true at C. The large vertical movement at C necessi- 

 tates a transfer of mass by viscous flow from below the mobile block, past 

 the lower end of the rupture CD, to the region to the left of C, and a 

 consequent rise of the surface at A. If A be a region of vigorous degra- 

 dation, this transfer of mass from the right of C to the left may be a 

 manifestation of isostatic adjustment and may in itself induce the gravi- 

 tative stress in the mobile block below CD which causes it to rotate. 



3. The third possible application of stress to a lower mobile block mov- 

 ing past an upper passive block is by the operation of a viscous current 



o 



Figure 6. — Diagram illustrating Thrusting by Rotation 

 AB, surface of the earth ; CD, curve thrust ; O, axis of rotation. 



below it. Here the stress would be applied to every point of the lower 

 surface of the block and would be non-uniformly distributed in its cross- 

 section. In the diagram, figure 7, let AB be the surface of the earth, CD 

 an existent lowly inclined rupture, and MN the boundary between the 

 rigid crust and an underlying viscous region. Let the current below MN 

 be from right to left. The strain in the prism CDNM will be torsional, 

 since the friction on CD is counter to the stress applied to MN. If the 

 strength of the rocks in this prism be sufficient to withstand the tendency 

 to rupture, and the current carry the prism with it, the latter will move 

 on CD against friction and an imbricated structure, due to failure, will 

 be developed in ABDC. The uniform application of stress at all points 

 on MN makes this possibility not improbable. If, however, CDNM fail, 

 the failure would occur in its lower part first because of the greater in- 

 tensity of strain in that region relatively to strength. As a result of fail- 



